I think you need a referral from your GP to the Paediatric department to try to get some advice.
You seem to have tried everything you can with no effect.
The paediatricians should either treat or rule out any physical problems, then , if necessary, refer on to the Child Psychologists.
If it is of any help to you, the son-and-heir ate only plain rice, plain pasta, peas, chips and Petit Filous (and an occasional tomato) from about age 3.5 until about age 13. He's now 25 fit and well enjoying his life. He didn't have the poo problems, though.